percpu: remove compile warnings caused by __verify_pcpu_ptr()
If percpu pointer is const, __verify_pcpu_ptr() triggers warnings like
the following.
drivers/net/loopback.c: In function 'loopback_get_stats':
drivers/net/loopback.c:109: warning: initialization discards qualifiers from pointer target type
Fix it by adding const to the verification target pointer used in
__verify_pcpu_ptr().
Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>